Completing the Picture: Determination of ¹³C Hyperfine Coupling Constants of Flavin Semiquinone Radicals by Photochemically Induced Dynamic Nuclear Polarization Spectroscopy

Abstract

We investigate the electronic structure of flavin semiquinone radicals in terms of their 13 C hyperfine coupling constants. Photochemically induced dynamic nuclear polarization (photo-CIDNP) spectroscopy was used to study both the neutral and anionic radical species of flavin mononucleotide (FMN) in bulk aqueous solution. Apart from universally 13 C-labeled FMN, partially labeled isotopologues are used to increase sensitivity for nuclei exhibiting very small hyperfine couplings and to cope with spectral overlap. In addition, experimental findings are supported by quantum chemical calculations, and implications for the spin density distribution in free flavin radicals are discussed.

Expanding Reaction Horizons: Evidence of the 5‐Deazaflavin Radical Through Photochemically Induced Dynamic Nuclear Polarization

“…The hyperfine coupling constants of the 5‐deazaFMN radical found can be compared to values of neutral and anionic FMN radicals already published [44,55] . The hyperfine structure of the FMN radicals differs significantly due to protonation of N5 and can thus be differentiated unambiguously by photo‐CIDNP measurements.…”

“…The hyperfine coupling constants of the 5-deazaFMN radical found can be compared to values of neutral and anionic FMN radicals already published. [44,55] The hyperfine structure of the FMN radicals differs significantly due to protonation of N5 and can thus be differentiated unambiguously by photo-CIDNP measurements. Comparison of 1 H hyperfine coupling constants shows that the 5-deazaFMN radical species resembles the anionic FMN radical whereas its hyperfine structure is hardly influenced by protonation of N1.…”

Erweiterung des Reaktionsspektrums: Nachweis des 5‐Deazaflavin‐Radikals durch photochemisch induzierte dynamische Kernpolarisation

“…Die so erhaltenen Hyperfeinkopplungskonstanten des 5‐Deaza‐FMN‐Radikals können mit bereits veröffentlichten Daten für neutrale und anionische FMN‐Radikale verglichen werden [44,55] . Die Protonierung von N5 lässt eine eindeutige Unterscheidung der beiden FMN‐Radikale durch photo‐CIDNP‐Messungen zu.…”

“…Die so erhaltenen Hyperfeinkopplungskonstanten des 5-Deaza-FMN-Radikals können mit bereits veröffentlichten Daten für neutrale und anionische FMN-Radikale verglichen werden. [44,55] In Bezug auf die Mulliken-Spinpopulationen von 5-Deaza-FMN *À und 5-Deaza-FMN(H1) * treten für 1 H-Kerne nur geringe Abweichungen auf (siehe Tabelle S5 und Abbildung S7). Ein Vergleich der Spindichteverteilung mit dem neutralen und dem anionischen FMN-Radikal, FMN(H5) * bzw.…”

Unravelling structures of radicals of kynurenic acid formed in the photoinduced reactions with tryptophan and N-acetyl tyrosine

“…When the HFCCs are not known from the EPR data, they can be calculated by the DFT method and used to identify radical structures by correlating geminate CIDNP signal intensities and calculated HFCCs. 14 Application of TR CIDNP can be expanded to the case of multiple radical pairs at the geminate stage recombining into a single pair of products. 15 As a result, TR CIDNP is a powerful tool for the identification of radicals in short-lived radical pairs in liquid solutions of low viscosity.…”